1. Field of Invention
This invention relates to new and improved microminiature structures for use as a field emitter electron sources and to new methods of manufacturing field emitter electron sources using semiconductor microminiature integrated circuit manufacturing techniques.
2. Background of Problem
It has long been known that electron current densities which can be obtained from field-emission sources is much greater than those that can be obtained from thermal sources. For example, field emission electron sources have been operated at current densitites as high as 10.sup.8 amperes per square centimer (10.sup.8 amps/cm.sup.2), while the maximum current density normally obtainable from a thermal electron source is less than 10.sup.2 amps/cm.sup.2. Additionally, field emitter electron sources intrinsically are smaller than thermal electron sources, and in general are less than one micron in diameter. In contrast, practical thermal electron sources cannot be made smaller than about 100 microns. Because there are many applications in which a small electron source size is important, this characteristic feature of the field emitter electron source makes it desirable for use in a number of equipments. For example, in high resolution scanning electron microscopy and in high density electron beam recording for information storage and retrieval, the field emitter electron source, because of its intrinsic small size, would be desirable.
In spite of the above-listed desirable advantages inherent with field emission electron sources, there has been no widespread practical use of such sources due principally to the limited lifetime which conventional field emission sources possess. It has been established that the primary cause of the short operating life characteristic of known field emission sources is due to the erosion of the emitting tip by ion bombardment of the tip by ions which are generated by the emitted electrons colliding with residual gas atoms normally surrounding the emitting tip.
There are a number of prior art microstructure field emission electron sources which have been developed in an effort to overcome this problem and are available to the industry. One such prior art microstructure field emission cathode is described in an article entitled, "A Thin-Film Field-Emission Cathode," by C. A. Spindt appearing in the Journal of Applied Physics, 39(7), 3504-05, June, 1968. Still another microstructure field emission cathode source is described in U.S. Pat. No. 3,453,478 -- Issued July 1, 1969 -- K. R. Shoulders, et al., entitled, "Needle-Type Electron Source," International Class HO-lJ, U.S. Class 313-309. The Journal of Applied Physics article describes a method for fabricating a microstructure field emission electron source which results in the formation of a single emitter tip at each one of a plurality of sites by the codeposition of a metal such as molybdenum at both normal and grazing incidence while at the same time rotating the substrate. This known technique is more complicated, expensive, and less likely to produce well-oriented single crystal tip emitters in a reliable and reproducible manner than is the present invention. The structure and techniques described in U.S. Pat. No. 3,453,478, results in the production of a multiplicity of emitter points at each field emission cathode site and is disadvantageous since focusing to a single fine spot with the multiplicity of emitter tips is difficult. To overcome these difficulties, the present invention was devised.